'''Nosology''' (in[[Greeklanguage|Greek]] ''Nosos'' = Disease) is a branch of [[medicine]] that deals with classification of [[disease]]s.

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'''Nosology''' (fromthe Greek "''nosos'',""disease") is a branch of [[medicine]] that deals with [[classification]] of [[disease]]s.

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Diseases can be classified by [[etiology]] (cause), [[pathogenesis]] (mechanism in which the agent causes disease) or by [[symptom]]. Alternatively, diseases can be classified according to the [[organ system]] they involve, but this is often complicated as many diseases affect multipleorgans.

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Diseases may be classified by [[etiology]] (cause), [[pathogenesis]] ([[mechanism]]by which the disease is caused), or by [[symptom]](s). Alternatively, diseases may be classified according to the [[organ system]] involved, though this is often complicated since many diseases affect morethan one organ.

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Oneofthemain problems withnosologies is that diseases often cannot be defined and classified clearly especially when pathogenesis or causalityis notknown. So, diagnostic terms often areinfactonly symptoms or sets of symptoms ([[syndrome|syndromes]]).

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Achiefdifficultyinnosology is that diseases often cannot be defined and classified clearly, especially when etiology or pathogenesisareunknown. Thus diagnostic terms often onlyreflectasymptom or set of symptoms ([[syndrome]]).

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One of the earliest efforts at developing a classification of diseases began in the [[10th century]], when the [[Islamic psychology|Arabian psychologist]] Najab ud-din Unhammad classified a nosology of nine major categories of [[mental disorder]]s, which included 30 different mental illnesses in total. Some of the categories he described included [[obsessive-compulsive disorder]]s, [[delusional disorder]]s, [[degenerative disease]]s, [[involutional melancholia]], and states of abnormal excitement.<ref>{{citation|title=Masters of the Mind: Exploring the Story of Mental Illness from Ancient Times to the New Millennium|first=Theodore|last=Millon|year=2004|publisher=[[John Wiley & Sons]]|isbn=0471679615|page=38}}</ref>

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In the 18th century, the taxonomist Carolus Linnaeus, Francois Boissier de Sauvages, and psychiatrist [[Phillipe Pinel]] developed an early classification of physical illnesses. [[Thomas Sydenham]]'s work in the late 17th century might also be considered a nosology. In the 19th century, [[Emil Kraepelin]] and then [[Jacques Bertillon]] developed their own nosologies. Bertillon's work, classifying causes of [[death]], was a precursor of the modern medical-billing code system, [[ICD]].

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The early nosological efforts grouped [[disease]]s by their [[symptom]]s, whereas modern systems (e.g. [[SNOMED]]) focus on grouping diseases by the [[anatomy]] and [[etiology]] involved.

Diseases may be classified by etiology (cause), pathogenesis (mechanism by which the disease is caused), or by symptom(s). Alternatively, diseases may be classified according to the organ system involved, though this is often complicated since many diseases affect more than one organ.

A chief difficulty in nosology is that diseases often cannot be defined and classified clearly, especially when etiology or pathogenesis are unknown. Thus diagnostic terms often only reflect a symptom or set of symptoms (syndrome).

In the 18th century, the taxonomist Carolus Linnaeus, Francois Boissier de Sauvages, and psychiatrist Phillipe Pinel developed an early classification of physical illnesses. Thomas Sydenham's work in the late 17th century might also be considered a nosology. In the 19th century, Emil Kraepelin and then Jacques Bertillon developed their own nosologies. Bertillon's work, classifying causes of death, was a precursor of the modern medical-billing code system, ICD.